Films made from crystalline silicon and amorphous silicon can be made with ultrathin thicknesses. Actually, it has been reported that a crystalline silicon layer can be formed with an average thickness of 0.7 nm by thermal oxidation of a SOI (Silicon on Insulator) substrate (see Non-Patent Literature 1). However, in such a thin film, it is difficult to control an interfacial structure thereof in an atomic layer level, whereby an interface state such as a defect or the like is caused in the thin film. Further, due to the ultrathin thickness of the ultrathin conducting layer, the interface state causes carrier scattering, thereby affecting conductivity in the ultrathin conducting layer. Actual mobility of electrons in the crystalline silicon having 0.7 nm in thickness is 50 cm2/Vsec. This is extremely inferior to 1000 cm2/Vsec or less that is mobility of bulk crystalline silicon. Furthermore, in such a thin film structure, conductive control by doping cannot be performed because a so-called dopant itself, such as phosphorus and boron, causes carrier scattering.
Amorphous silicon is a semiconducting material used for a TFT of liquid crystal, a solar cell and the like. However, the amorphous silicon has lower mobility than crystalline silicon whose mobility is 1 cm2/Vsec for n-type semiconductor and the mobility of the crystalline silicon is 0.1 cm2/Vsec for p-type semiconductor. Therefore, the amorphous silicon cannot be used for a material for a high-performance device such as a LSI. Further, the amorphous silicon requires a hydrogenation of dangling bond so as to retain properties as a semiconducting material. However, it has been known that, because the end of the dangling bond is dehydrogenated by hot electron or photoirradiation, a device having the amorphous silicon falls into out of order.
There has been known that organic materials, such as a pentacene, encompass a material having mobility higher than the amorphous silicon. The material has been regarded as a potential material for organic devices such as an electroluminescence, a field effect transistor and a solar cell (see Non-Patent Literature 2).
However, electron field-effect mobility of electron holes of p-type pentacene is no more than 0.75 cm2/Vsec (see Non-Patent Literature 3). Therefore, the electron field-effect mobility of the electron holes of the p-type pentacene is inferior to the mobility of the crystalline silicon. As a result, the p-type pentacene cannot be a material for the high-performance device. Further, the p-type pentacene has lower heat resistance and oxidation resistance than those of silicon materials. The p-type pentacene is also inferior in stable performance. Particularly, the p-type pentacene has a melting point of substantially 300° C., and therefore, cannot be used for a device that would reach a high temperature.
A patent (see Patent Literature 1) discloses a cluster compound of a transition metal and silicon. However, the patent does not disclose an array of the cluster compound.
Another patent (see Patent Literature 2) discloses that a semiconductor is made of a laminated substance in which a transition metal-containing silicon cluster having a MSi12 composition is arranged. This patent also discloses a semiconductor device that uses the laminated substance. However, the semiconductor device is not sufficiently functional because the laminated substance has a narrow band gap.
Paten Literature 1
Japanese Patent Application Publication, Tokukai, No. 2000-327319 A
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2008-28125 A
Non-Patent Literature 1
IEEE International Electron Devices Meeting 2003 Technical Digest, p 805
Non-Patent Literature 2
Journal of the Physical Society of Japan 2007, vol. Nov., p 851
Non-Patent Literature 3
Advanced Materials 2007, vol. 19, p 678